It has been established that weak interfaces are desirable in ceramic composites between the reinforcing fibers and the ceramic matrix material to attain toughening from the fiber reinforcements over a wide range of temperatures. An unbonded or weakly bonded interface allows sliding between the fibers and the matrix, and/or preferential crack deflection around the fibers, for optimal toughening of the composite. Although composites containing layers of carbon or BN at the fiber/matrix interface have been developed, there are no weakly bonded composites known in the prior art that are stable in very high temperature, oxidizing environments. Previous work has shown that it is difficult to find suitable composite systems comprising a ceramic matrix, fibers having high strength and high Young's modulus, and a weakly bonded interface material, all of which exhibit long term compatibility in high temperature oxidizing environments. Furthermore, most suitable fibers and matrices are multiphase materials. This generally reduces the compatibility of the materials, particularly over a range of temperatures, and increases the complexity of chemical processing. The use of barrier layers to separate incompatible materials is undesirable because it adds to the complexity of the system and only postpones unwanted chemical reactions. Thus, there is a need for new high temperature ceramic composites that have a weakly bonded interface between reinforcing fibers and matrix materials and that are thermodynamically stable in oxidizing environments at temperatures up to approximately 1800.degree.-1900.degree. C.